A more effective prevention/treatment is needed for major depressive disorder (MDD), a leading disease burden. Enriched environment treatment (EET), which includes physical exercise, mental stimulation, and social interactions, is a potential intervention to prevent/treat MDD. The neurotrophic effects of EET have been extensively studied; however, its antidepressant effects and underlying biological mechanisms are unclear. In particular, its age-dependent effects and (epi-)genetic mechanisms remain to be established. Identifying the life period for maximal EET effects and its antidepressive mechanisms is important in helping develop strategies for a more effective prevention/treatment of MDD. Our long-term goal is to clarify age x (epi-)genetic interference of the antidepressive effects of EET. This project will specifically aim to clarify the antidepressive effects of early-lfe EET, focusing on long-lasting expression changes of brain-derived neurotrophic factor (BDNF; a major neuronal growth factor in the brain related to MDD) and depression-related genes. BDNF deficiency, particularly, Bdnf promoter IV deficiency caused by epigenetic regulation, has been observed in MDD patients and stressed animals. Early-life maltreatment of infants results in DNA methylation of the promoter IV-controlled exons and reduces BDNF expression throughout life. Abusive maternal behavior and previously acquired DNA methylation patterns then transmit perpetually from generation to generation. We recently showed causal evidence whereby Bdnf promoter IV deficiency leads to depression-like behavior. Further, using our promoter IV-deficient depression-model mice (KIV), we showed that chronic EET, but not chronic antidepressant treatment, was able to compensate for the reduced BDNF levels caused by promoter IV deficiency through multiple promoter-driven BDNF, which paralleled antidepressive behavioral effects of EET. Since early-life experiences involve dynamic gene expression regulations, we hypothesize that the antidepressive effects of EET and Bdnf compensation mechanisms may be maximized when EET is provided during early-life development and that these effects will endure in later life due to long-lasting expression changes of Bdnf and Bdnf-/depression-related genes. We will test these hypotheses with two aims: Aim 1) to determine antidepressant, Bdnf, and gene effects of early-life EET, and Aim 2) to examine whether these effects of EET endure when EET is provided during early-life development. In Aim 1, we will determine the effectiveness of EET across ages-during early-life and at two (young or old) adult stages-in both normal and depressed (KIV) mice, by measuring i) depression-like behavior, ii) BDNF levels, and iii) expression changes in Bdnf-/depression-related genes in the brain regions related to MDD. We will use a novel high-throughput gene analysis. In Aim 2, we will determine prolonged effects of 8 weeks of EET after a subsequent 4 weeks of standard condition treatment, by measuring i-iii as in Aim1. Once this project is completed, we will further clarify te precise critical period for effective EET and the gene mechanisms underlying depression resistance.